If you've got a big idea, there's no doubt that you will run into skeptics along the way.
Many skeptics are afraid for you, embrace the status quo, and in their twisted but well-intentioned way, will work to persuade you to give up your dream. This sort of skeptic should be ignored, certainly. It doesn't really pay to argue with them, because your impassioned restatement of your view of reality will do little to persuade them that you're not doing something crazy risky.
The other kind of skeptic, though, should be treated totally differently.
The generous skeptic has insight into your field, your strengths and weaknesses. She wants you to succeed, but maybe, just maybe, sees something you don't.
When the generous skeptic speaks up, she's taking a risk. If you respond to her generosity by arguing, by shutting down, by avoiding eye contact or becoming defensive, you've blown it. You've taken a gift and wasted it, and disrespected the gift giver at the same time.
The alternative is to emotionally stand up and sit down on her side of the table. Egg her on. Imagine the world the way she sees it. Take her tactical skepticism and amplify it, pushing it to its logical conclusion. Instead of defending the flickering flame of your idea as if it might soon be extinguished, dump as much of this sort of skepticism on the idea as you can.
Not only are you honoring the generous skeptic when you do this, you're learning how to see the way she sees. Your job isn't to persuade her she's wrong, your job is to learn from this and buttress your project in a way that when it collides with the market, you're ready.
"Tell me more about that," is the useful and productive response, not, "no, you're wrong, you don't understand."
There's always time to ignore this feedback later. Right now, dive into it, with an eager, open mind. It's a gift you're not often offered.